vb.net 在组合框上填充文本框选定的索引已随 sql 数据库更改 [英] vb.net Filling Textbox On combobox Selected Index Changed with sql database

问题描述

在我编写此代码以在组合框选择索引更改时填充文本框后

after i programming this code for fill text-box when combo-box selected Index changed

我收到此错误未将对象引用设置为对象的实例."，我能做什么??

i got this error 'Object reference not set to an instance of an object.', whats can i do ??

Private Sub participant1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles participant1.SelectedIndexChanged

    Try
        Dim cmd As SqlCommand = New SqlCommand()
        Dim datareader As SqlDataReader = Nothing
        If Class1.sqlcon.State = ConnectionState.Open Then
            Class1.sqlcon.Close()
        End If
        Class1.sqlcon.Open()
        Dim query As String
        query = " select * from tparticipant where namea = '" & participant1.Text & "'"
        cmd = New SqlCommand(query, Class1.sqlcon)
        While datareader.Read
            If datareader IsNot Nothing Then
                ID.Text = datareader.GetInt32("ID")
                total.Text = datareader.GetInt32("total")
            End If

        End While

    Catch ex As Exception
        MessageBox.Show(ex.Message)
    End Try`

推荐答案

您需要将 cmd.ExecuteReader() 分配给您的 datareader.这就是您的 NullReference 的来源.

You need to assign the cmd.ExecuteReader() to your datareader. Thats where your NullReference is coming from.

datareader = cmd.ExecuteReader()

这篇关于vb.net 在组合框上填充文本框选定的索引已随 sql 数据库更改的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！